FIG. 1 illustrates the essential components of the classical wound-rotor induction motor with slip rings known in the prior art. FIG. 2, also prior art, shows the motor torque, current, and load torque versus speed curves at different rotor resistance r.sub.2 for the wound-rotor induction motor of FIG. 1. A typical fan-type load torque curve intersects the motor torque curves for different rotor resistances at points A, B, and C. The speed and the current are lower for higher rotor resistance r.sub.2.
FIG. 3, also prior art, shows the equivalent circuit of the motor of FIG. 1. It is similar to the equivalent circuit of a transformer. In fact, an induction motor can be viewed as a transformer. The stator winding is the primary winding and the rotor cage is the secondary winding of a transformer. In FIG. 3, although the stator and rotor are separated by an air gap, the rotor resistance r.sub.2 and the rotor leakage reactance x.sub.2 can be viewed as electrically connected to the circuit that comprises the stator resistance r.sub.1, stator leakage reactance x.sub.1, magnetization reactance x.sub.m, and the core loss resistance r.sub.fe. This is because the rotor and stator assembly are magnetically coupled to each other through the air gap. The symbol s is the motor slip. It represents the relative difference between the motor synchronous speed and the actual motor shaft speed. The motor mechanical output can be obtained by computing the energy associated with the equivalent rotor resistance r.sub.2 over the slip s minus the rotor copper loss.
Over the years, adjustable speed electric motor drive packages have been achieved in various ways in different motor and control designs, including that shown in FIG. 1. (The electric motor drive package is combination of the motor and its control system.) In FIG. 1, for example, it is known to tap the rotor currents with brushes and route the rotor currents through an external resistor bank or energy conversion circuit. Variable resistor banks and energy conversion circuits allow smooth speed changes with the motor of FIG. 1.
Other adjustable speed electric motor drive packages, however, are characterized by abrupt speed changes. Examples include multiple speed winding motors such as the 2:1 ratio dual winding motor, and pole-amplitude-modulated (PAM) motors that give a synchronous speed ratio other than 2:1.
Adjustable frequency electric motor drive packages have also been developed. They tend to be expensive because they require fully-rated power electronic converters and inverters.
A drawback of most of the above electric motor drive packages is that when a power electronic control circuit is used, it has to be fully power-rated. By this is meant the power electronics needs to be capable of handling at least the full motor output power rating. Full power rating in the electronics drives up the cost of adjustable-speed electric motor drive packages, making them very expensive. The motor control system costs as much as three times that of the motor itself.
A recent development which lowers the cost of an adjustable-speed electric motor drive package is the brushless doubly-fed induction motor (BDFIM). One example, described by Liao (1996), has two sets of stator windings of 2p-pole and 2q-pole. The rotor winding has a 2(p+q)-pole nested cage. It produces less power output for a given frame as compared with the power output of a conventional motor. The BDFIM significantly lowers the electric motor drive package cost because only a portion of the motor output power has to be fed through its power electronic control circuit. That is, unlike many other adjustable-speed electric motor drive packages that require fully rated power electronics inverters and converters, the BDFIM requires only a partially rated power electronics inverter and converter.
However, since the BDFIM uses two different numbers of poles for its windings, it generates more harmonics than motors using a single number of poles. It would be desirable to eliminate these harmonics because all harmonics produce energy losses.
Thus, while the BDFIM makes possible an adjustable speed electric motor drive package of lower cost than other adjustable speed electric motor drive packages that have fully-rated power electronics inverters and converters, it produces unwanted energy-wasting harmonics. The utilization of motor active material (laminations, windings, etc.) is also less effective for motors with 2-p and 2-q poles than for a motor with windings of a single number of poles.
In contrast to the prior art, the present invention is a low-cost electric motor drive package based on a new motor design that produces no unwanted harmonics, is not a conventional BDFIM, yet can be used with a partially-rated power electronics control system resulting in a lower system cost.